Sound-transmission, sound-recording, and sound-reproducing system



37- A. D. BLUMLEIN 2,098,372 I SOUND TRANSMISSION, SOUND RECORDING, AND SOUND REPRODUCING SYSTEM Filed Oct. 31, 1934 v 2 Sheets-Shet- 1 J J H F 8 10 7 T Ell/410416 v [N wax/TOR z www Nov. 9, 1937. A. D. BLUMLEI N 2,098,372

SOUND TRANSMISSION, SOUND RECORDING, AND SOUND REPRODUCING SYSTEM Filed Oct :51, 1934 2 Sheets-Sheet 2 Zhwento:

Patented Nov. 9, 1937 UNITED STATES SOUND TRANSIWISSION, SOUND RECORD- ING, AND SOUND-REPRODUCING SYSTEM Alan Dower Blumlein, West Ealing, London, England, assignor to Electric & Musical Indus tries, Limited, Blyth Road, Hayes, England Application October 31, 1934, Serial No. 750,898 In Great Britain October 23, 1933 2 Claims.

This invention relates to systems for the electrical transmission of sound, and is an improvement in or modification of the invention described in the specification of application No.

647,057 filed the 13th day of Deceinber, 1932. This prior specification describes transmission systems (including also, if desired, recording and/ or reproducing stages) wherein impulses are picked up by a plurality of microphone elements and reproduced by a plurality of loudspeakers; and there are described therein means for controlling or modifying the impulses so that the sounds heard from the loudspeakers will convey to a listener a locational impression of the sound source. One of the arrangements set forth in the above-mentioned specification for obtaining this efiect and described in "part more fully below 20 comprises the use of two pressure microphones, in

and network arrangements of electrical elementsin the transmission circuits from these microphones, whereby phase difierences between the microphone outputs are converted into difler ences of loudness between the loudspeaker outputs. The step of .modifying the impulses to obtain the desired effect will be referred to in this specification as "shufliing, and the networks employed for this purpose as shuflling networks.

In connection with the arrangement referred to difliculties have been encountered in dealing with sound waves of low and very low frequencies in order to provide the effect at which the invention aims. As explained in the earlier specification the apparent direction from which the sounds reach the microphones (as determined by the relations between the impulses in the microphones) becomes ambiguous if the distance between the microphones exceeds half the wave length of the sound waves in question, and in order therefore that they may deal satisfactorily with an adequate range of frequencies (and in particular receive unambiguously the middle and high frequencies of the range) it is desirable to place the microphones close together. If this is done, however, the phase difference between the microphone outputs at low and very low frequencies (from an obliquely situated sound source) be-- comes very smalls Theoretically this state of renders it necessary to make the microphones and associated circuits correctly sensitive to very small phase variations and differences, and this involves the use of large amplification (with the attendant risk of parasitic noises), and of transformers and similar apparatus designed to introduce negligible, or accurately controlled, phase changes at low frequencies. The usual types of speech range transformers employing a; lami- 60 nated magnetic core produce large phase shifts spaced relationship, forpicking up the sounds affairs is not unsatisfactory, but in practice it at low frequencies and these phase shifts are diflicult to control owing to the variable nature of the transformer cores. V i

To overcome these 'difliculties it is proposed in this invention to use two pairs of microphones, one pair dealing with the lower frequencies and the other with the higher frequencies. The separation of the microphones canthen be suitable for the range of frequencies dealt with by the microphones, and is not the compromise necessary when one pair of microphones deals with the whole frequency gamut. The outputs of a pair of microphones are dealt with separately and shufiled so that a binaural effect is obtained from this pair; and similarly with the other pair. The four channels are then combined so that two loudspeakers each reproducing the whole frequency range may be used.

The invention'will be more readily understood from the following description of a few forms thereof, but it must-be understood thatthese are described only by way of example. The understanding of this description will be facilitated by reference to the accompanying drawings wherein Figure 1 represents in usual symbols an electrical circuit forming one embodiment of the invention, and

Figures 2 to 6 represent detailsof arrangements suitable for the embodiment shownin Figure 1 or for various other embodiments of the invention.

Figure 1 showstwo pairs of pressure microphones I, 2, 3 and 4, arranged symmetrically about the center line or axis of the sound field, one pair I and 2 being widely spaced from one another and the other pair 3 and 4 being closely spaced. Separate shuflling networks 5 and 6 are provided, one for each pair of microphones, and

these operate to combine the outputs of the Figure 4, the impulses ,in the sum channel are modified in amplitude only by the resistive T- network comprising series resistances'g and h This 7 process is symbolized in Figure 3. As shown in and shunt resistance 1'. The impulses in the difference channel are modified in amplitude and changed in phase'by by means of the net-.

work comprising-series resistances d, e and a shunt condenser j. The resistance R: in series with the condenser is inserted if it is not desired to change phase difierences into amplitude difierences over the whole of the frequency range of the pair of microphones. Above a certain frequency in the range, therefore, at which the impedance of the condenser is comparable with that of the resistance, thev phase change is obtained to a smaller degree. The voltages at the output of the two networks are now substantially in phase and of diiferent amplitude. A further sum and difference operation is performed by a second transformer represented in Figure 4 by the symbol of Figure 3. The final channel voltages are now in phase, but differ angle will be small, so that cos p Current from right-hand microphone is defined by A cos (wt-45) V Where A=amp1itude of an impulse from either microphone:

w=the angular frequency of the transmitted impulses:

t=the time, measured from the commencement of the impulses:

and =half the phase difference between impulses from the two-microphones.

The above expressions represent instantaneous conditions at any time during transmission.

Semi-sum of above= 2 o cos Acoswtcosd: Semi-difference= c (Hos -A sin a! sin 45 The electrical phase of the above two depend on the terms in wt only and it will be seen that the two waves are 30 out of phase.

. The sum channel is now modified by a multiplier a, and the diflerence by a multiplier (integration).

These factors are introduced by the modifying networks shown in the two channels, and 7' is the usual symbol representing the imaginary quantity 1, well known in this art.

The resultant waves will be:- Modified sum=aA cos mtcos gA cos wt sin The final summing and differencing will give V Left hand channel (aA cos 5A sin rt) cos w t Right hand channel (aA cos 4)- 3A simfi) cos mt 0 where c is the velocity of sound. This is equivalent to an electrical phase difference of the two microphone outputs of This phase difference is equal to 21. Now if the wave length is long compared with 1 this phase will equal unity and sin will equal wy sin 9 v 20 The expression for the left hand channel output now becomes:-

(aA+gA- g) c0s wt=A (a+ -':sin 9) cos wt and similarly the expression for the right hand channel is the same with the sign reversed. From the above it will be seen that the ratio of the amplitudes of the two channels is independent of m, but depends only on 9, the direction of arrival of the incident sound.

Referring again to Figure 1, it will be seen that impulses from a pair of microphones (say micro-'- identical with that shown in Figure 4 described above, and from the above analyses it will also be seen that, on account of the two summingand-differencing operations with the intervenin modifying networks in the shufller, the impulses further transmitted to the loudspeaker channels, I l and I2, are in phase and differ in amplitude by an amount depending solely on the orientation of the sound source with respect to the line joining the microphones. Similarly the impulses from shufller 5 originating from microphones l and 2 are in phase, and differ in amplitude by an amount depending upon the position of the sound source-relative to the microphones. This is a condition required in the case of each pair of microphones, accordi'ngto the invention in order to provide a'binaural sensation to a person listening to the loudspeakers. A

It will'be clear from the above analysis that by p choosing the value of the shunt resistance i in t the summation channel and the shunt condenser j in the difference channel for a given frequency, any degree of amplitude difference in the final channels can be obtained for a given phase diflerence in theoriginal channels. For the low frequencies it can be shown that the phase diiference between the waves will, for a given obliquity of the sound source, vary proportionately with frequency, being very small for a very low frequency. Thus, for a given obliquity of the sound the current in the difference channel after the first sum and difference operation will be increasingly great compared with that in the summation channel the higher the frequency. Hence the use 019. shunt condenser f where impedance decreases with frequency in the difference circuit will have the desirable effect of producing a fixed intensity difference in the final channels for a given obliquity at alllow frequencies, as well as the.

Referring again to Figure l, 5 and 6 represent each a complete shufliing network such as illustrated for example in Figure 4, and while 5 converts phase differences between microphones l and 2 into amplitude differences in the output H channels, 6 operates in the same way upon impulses from microphones 3 and 4.

From the explanation given above it' will clear that the closely spaced microphones 3 and 4 while being suitable for dealing with middle and high frequencies are not entirely satisfactory as regards low frequencies on account of their closeness to one another, while the widely spaced microphones I and 2 although convenient for low frequencies introduce ambiguity in the directional impression which they'provide as regards medium and high frequencies. For this reason the closely spaced microphones 3 and 4 and their associated shufliingnetwork 6 are arranged to deal mainly or entirelywith medium and high frequencies and no attempt is made to obtain separation of the low frequencies. The widely spaced microphones l and 2 and their associated shuiiiing network 5 on the other hand are arranged to deal mainly or entirely with low and very low frequencies, but no separation of. the higher frequencies is obtained. The arrangements employed for rendering the microphones operative over portions of the frequency range may be of any convenient kind sifch, for example, as those which have been used for a similar purpose in connection with loudspeakers, and may comprise adaptation of the mechanical characteristics of the microphone, and/or of the electrical characteristics of the microphones or their circuits (e. g. by adding filter networks).

The voltages from the shirfliing networks transformed or assumed transformed to a common impedance (of value R) are fed (after further amplification if necessary) to two loudspeakers which are common to the two networks.- Two pairs of leads, as shown, connect each shufliing network one to each loudspeaker and the pairs of leads from the high frequency shufiling net-i work 6 (i. e. the network associated with the closely spaced microphones) are, in each case, shuntedby an inductance Band III, while the pairs of leads from the low frequency shufiiing network 5 are in each case shunted by a condenser "I and 8. The leads to each loudspeaker are connected to the outer terminals of the appropriate inductance and condenser, which thus remain in series as a shunt across the loudspeaker. Thus condenser I and inductance 9 are in series with one another as a shunt across the leads II going to one loudspeaker while condenser 8 and inductance Ill in series form another shunt across the leads 12 to the other loudspeaker. The relative values of the inductances and capacities to be employed are determined by the equation where Lrepresents the value of the inductance,

These additional microphones operate, and are C the value of the capacity and R the value of the impedance (referred to above) in any consistent system of units.

It is to be understood that frequency discriminating networks may be employed having any 5 convenientform, and may be quite different from those shown. For example, sharp selective highpass and low-pass filters may be used, or any other electrical change-over circuit. Such frequency discrimination elements may be inserted within the shuiilers and the channels may be mixed if desired before the final summing and differencing operation. It is, however, preferable that the discriminating networks which apply to the two channels be similar to one another. 1

The sense of poling of the connections is such that in a uniform fface on sound field the contribution from the widely spaced microphones I and 2 is added to that from the closely spaced microphones 3 and 4 in the correct phase at the points of junction so that a uniform response to such a wave is obtained throughout the full frequency range. In order to achieve this, it is necessary to arrange that the relative efiiciencies of the portions of the system between a, microphone and the .output terminals of its shuiiiing network shall be of the same value in each case. The values of the inductances and condensers are chosen so that low frequency impulses are received from the widely spaced microphones, and the medium or high frequency impulses from the closely spaced microphones.

It be clear that a number of modifications maybe i troduced into a system of the character described above. For example, in place of two pairs of -microphones three pairs may be employed, the third pairbeing of very small size and placed very near together (between the pair 3 and 4 previously referred to 'as closely spaced).

the shufiled outputs from these additional, very high frequency microphones may be shunted by small inductances, these added condensers and inductances being in series as before and acting as a shunt across the final loudspeaker leads Which are connected to their outer terminals.

In any of the systems described above any pair or pairs of microphones may be replaced by directionally sensitive microphones or microphone elements (e. g. velocity microphones), the outputs of which will not generally be shumed to convert phase differences to amplitude differences (although this may be effected if desirable), but modifications may be eflectedto increase or de- Such arrangecrease the amplitude differences. ments for using directionally sensitive micro phones are described in the abovementioned specification No. 641,057 filed the 13th day of December, 1932. If a pair of pressure mi rophones be replaced by velocity microphones the spacing between them need not be maiained, and the velocity microphones may be rought close together. For example in the systems described above the low and middle frequency sounds may be picked up by one or two pairs of pressure microphones provided with suitable shuiliing networks, and the high frequencies by two velocity microphones. or microphone elements arranged close together, or (e. g. in the case of strip microphones) in line one with the other, their axes of maximum sensitivity being suitably inclined to one another in the manner described in the abovementioned specification.

An arrangement of this kind is illustrated in Figures 5 and 6 extracted from said priorspecification. In Figure 5 n and 0 represent in plan the strip elements of two velocity microphones placed one above the other in line perpendicular to the paper. Since these strips are colinear with one another no phase difierences between their outputs occur, but as a sound source moves from the centre of the sound field s to a position 11 the diiIerence between the 'amplitudesof their outputs varies with direction of the sound source. When these amplitude differences are to be augmented, as explained in the earlier specification a shufliing network of the kind shown in Figure 6 is appropriate. This is substantially identical with that shown in Figure 4 except that the shunt condenser j and resistance is in series, and the shunt resistance 1' are replaced by shunt resistances l m which are preferably variable as shown. These lines therefore form artificial attenuators and by altering their relative attenuation the intensity diflerences in the two lines corresponding to a given obliquity of sound are controlled. 1

The invention is not limited solely to the. arrangements described .above since various modifications may be introduced as they become necessary or desirable in order to carry the invention into effect under different conditions or requirements which have to be fulfilled withoutdeparting in any way from the scope of the invention.

Having now described my invention, what I claim as new and desire to secure by Letters Patent is:- r

1. In a system of sound transmission comprising a plurality of microphones connected to a.

plurality of loud speakers, two comparatively closely spaced microphones adapted to pick up high frequency sounds, two more widely spaced microphones adapted to pick up sounds of lower frequencies, and means in the circuits from each 

